1. Field of the Invention
The present invention relates to a manufacturing method of a silicon carbide semiconductor device.
2. Description of the Related Art
Research and development of next generation semiconductor devices that use a semiconductor substrate (silicon carbide substrate) employing silicon carbide (SiC) as a semiconductor material is advancing. Similar to silicone (Si), silicon carbide can form a gate insulating film by thermal oxidation. However, silicon carbide has a characteristic in that differences in channel mobility near the junction interface (hereinafter, MOS gate interface) of the silicon carbide substrate and the gate insulating film forming the metal oxide semiconductor (MOS) gate occur depending on the surface orientation of a principal surface of the substrate, the thermal oxidation method, etc. Interface state density is used as an index to alternatively evaluate channel mobility. In general, it is known that the smaller the interface state density is at the MOS gate interface, the greater the channel mobility tends to be.
As a method of forming a gate insulating film on a silicon carbide substrate, a method of performing thermal oxidation using an atmosphere that includes nitrogen such as nitrous oxide (N2O), nitric oxide (NO), etc. is known. Under this method, oxidation and nitriding simultaneously occur and nitrogen atoms in the atmosphere contribute to termination of dangling bonds of silicon atoms of the MOS gate interface and in the gate insulating film, thereby achieving a lower interface state density at the MOS gate interface. Further, immediately after this oxidation, annealing (heat treatment) in an atmosphere that includes hydrogen (H2) is performed and dangling bonds of the silicon atoms remaining after oxidation are terminated by hydrogen atoms, whereby the interface state density at the MOS gate interface is further decreased.
In the manufacturing process of a semiconductor device that uses a silicon carbide substrate, a process of heat treatment is essential after the formation of the gate insulating film, such as heat treatment for lowered resistivity and formation a gate electrode, heat treatment for baking and the formation of an interlayer insulating film, and heat treatment for the formation of a reactive layer (electrical contact unit) of a silicon carbide semiconductor (silicon carbide substrate) and a contact metal, and the formation of the contact metal. In the formation process of the gate insulating film, interface characteristics such as the interface state density at the junction interface of the silicon carbide substrate and the gate oxide film are set such that given conditions are satisfied, however, the interface characteristics are known to change consequent to heat treatment after the formation of the gate insulating film (for example, refer to Japanese Laid-Open Patent Publication No. 2007-242744).
Japanese Laid-Open Patent Publication No. 2007-242744 proposes a method in which in the manufacture of a metal-oxide-semiconductor field-effect transistor (MOSFET) having a MOS gate on a (000-1) surface of a silicon carbide substrate, heat treatment for forming a reactive layer of a contact metal and a silicon carbide semiconductor is performed in a mixed gas atmosphere (forming gas (FG) atmosphere) of hydrogen and an inert gas, rather than in an inert gas atmosphere, whereby increases in the interface state density at the MOS gate interface are suppressed and contact resistance is reduced.
Nonetheless, as described above, in a semiconductor device that uses a silicon carbide substrate, high-temperature heat treatment is essential after the formation of the gate insulating film and consequent to the conditions of this heat treatment (temperature, atmosphere, etc.), the characteristics of the semiconductor device change. Therefore, in addition to formation conditions of the gate insulating film, conditions of the heat treatment performed after the formation of the gate insulating film have to also be considered. For example, the semiconductor device is preferably in an OFF state (hereinafter, normally OFF) when no signal is being input to the gate. In other words, the threshold voltage of the semiconductor device is preferably a positive value.
The threshold voltage is expressed by a function having the flat band voltage as an argument and to make the threshold voltage a positive value, the flat band voltage is preferably a positive value. Nonetheless, in the described formation method of a gate insulating film, i.e., in a method of performing thermal oxidation of a (000-1) surface or a (11-20) surface of a silicon carbide substrate in a gas atmosphere that includes nitrogen such as nitrous oxide or nitric oxide and further performing annealing in an atmosphere that includes hydrogen to reduce the interface state density at the junction interface of the gate oxide film and the silicon carbide substrate, the flat band voltage becomes a negative value and when no signal is being input to the gate, the semiconductor device is in an ON state (normally ON).